We have learned a great deal recently about the mechanism and biology of a class of small non-coding RNAs called microRNAs. This remarkable class of RNAs constitute 2% of the genes in the human genome, and they repress the expression of protein-coding genes. This is achieved in animal specieis primarily by attenation of protein synthesis from mRNA transcripts which contain complementary sequence in their 3' untranslated regions. Although it is impossible yet to accurately estimate the extent of microRNA regulation, up to 30% of protein- coding genes are thought to be directly controlled by microRNAs. Genetic studies in model organisms support the notion that microRNAs play diverse roles in cell and organismal biology. We are interested in understanding the biological consequences of regulation by microRNAs. To this end, we have developed genetic and cell biological methods to study individual microRNAs in whole Drosophila tissues and cells. We will evaluate the mechanisms by which microRNAs buffer gene regulation against environmental perturbation such as temperature fluctuation. This is important for differentiation and development of the nervous system. The upstream regulatory pathways that regulate the process will be evaluated, and correlated to known biological functions of these pathways. We are also interested in the evolutionary implications of genetic buffering by microRNAs. We will experimentally test whether microRNAs buffer traits under selective pressure upon animal populations. Finally, we will evaluate the mechanisms by which microRNAs regulate cell morphogenesis in epithelial tissues, using a combination of quantitative experimental methods and theoretical physical modeling. The significance to health is manifold. The combined effects of microRNAs affect the expression of many human genes, and misregulation of microRNAs appear to underlie diverse disease phenomena such as cancer and virus infection. PUBLIC HEALTH RELEVANCE: MicroRNAs are regulators of gene expression, and their dysfunction is linked to diverse forms of cancer. Additionally, microRNAs play important functions during infection by numerous viruses. This project is to understand how microRNAs control gene expression and their natural functions within the body.